musicxml2ly bug: Fails on empty <work-title /> node

[James Harkins]

In case anybody is working on musicxml2ly:

I just found that it doesn't handle the case of an XML document where the 
title node exists, but is empty. I've uploaded a zip archive of two 
musicxml documents.

http://ubuntuone.com/0iNRBh17UxPJWlVJikFYVD

** 13-1220-li-xuanyi.xml -- The original file from my student, exported 
from Sibelius on Mac.

$ musicxml2ly 13-1220-li-xuanyi.xml 
musicxml2ly: Reading MusicXML from 13-1220-li-xuanyi.xml ...
Traceback (most recent call last):
 File "/usr/local/lilypond/usr/bin/musicxml2ly", line 2984, in ?
   main()
 File "/usr/local/lilypond/usr/bin/musicxml2ly", line 2979, in main
   voices = convert (filename, options)
 File "/usr/local/lilypond/usr/bin/musicxml2ly", line 2881, in convert
   score_information = extract_score_information (tree)
 File "/usr/local/lilypond/usr/bin/musicxml2ly", line 201, in 
extract_score_information
   set_if_exists ('title', movement_title.get_text ())
AttributeError: 'NoneType' object has no attribute 'get_text'

** 13-1220-li-xuanyi-finale.xml -- Another version, made by importing the 
original into Finale 2010 and re-exporting to musicxml. This one is fine in 
musicxml2ly, producing a LilyPond file that compiles successfully (apart 
from LilyPond complaining about all the collisions that Finale 
generated...).

A notable difference is that the one from Sibelius includes the following, 
but Finale omits it.

<work>
 <work-title />
</work>

If I remove those lines, then musicxml2ly gets past this error... only to 
fail later. So this document (the original) might be a good test case for 
musicxml2ly's error recovery.

I'm on 2.16.2.

Thanks,
hjh